At the present time, there are no processes which are generally suitable for continuously producing rubber mixtures. This is due to the wide variations in the number of different constituents utilised for forming the mixture, in the metering characteristics of such constituents and the physical form in which they are supplied. A continuous process, known as the Buss process, is known but this can only be used for the processing of powdered rubber.
In such known process, the constituents are fed, in a metered manner, into a single-screw extruder at various points along the extruder. The flight of the screw is perforated and the screw itself performs a vibratory movement in an axial direction in order to improve the negative conveying characteristic necessary in a mixing process and, hence, to improve the dwell time behaviour in a screw conveying means. Although this compensates for fluctuations in the metering of the components and also compensates for local accumulations of individual constituents, it results in a locally differing, volume-related loss of efficiency. This causes a non-uniform dispersion of the additives, such as carbon black, in the mixture, poor heat transference and hence locally different degrees of mixing of the additive in the material. Despite the axial movement of the worm, the constituents are still inadequately intermixed axially during the mixing process, and such process cannot compensate for these differences in the mixing state in different regions of the extruder. This known process cannot be used for producing rubber mixtures containing a high content of carbon black and/or non-pulverulent rubber, nor can it be used for making mixtures containing natural rubber.
The mixing process described hereinbefore cannot be improved simply by using different screw constructions normally used in profile extrusion, in which the individual layers of the fluidised components are rearranged using various geometrical constructions and physical barriers. Specific constructions of this kind do not achieve any crucial change in the dwell-time characteristic of the mixture constituents.
Accordingly, rubber-containing mixtures have hitherto been manufactured almost exclusively by discontinuous processes. Internal mixers can be considered to be the best known type of apparatus used for carrying out such a process. The polymeric compound and, at predetermined intervals of time, the other constituents of the mixture, are introduced into the internal mixer, and the mixture is kneaded therein for a preselected period of time.
Owing to the geometry of the rotor and the constantly changing redistribution of the components, uniform homogenisation of the components of the mixture is achieved during such kneading operation. One disadvantage is, however, the unfavourable material volume-to-mixer surface area ratio and the resultant poor heat transfer. If, for example, carbon black is to be incorporated in the mixture, a plurality of such mixing operations is necessary. This is because the mixture becomes heated to an unacceptable level if a single-step operation is attempted. Accordingly, the carbon black is added in batches in a plurality of mixing operations.
A flow diagram of the process in accordance with the prior art is shown in FIG. 10. In this example, mastication of the rubber is effected in an intermittent process in an internal mixer, under constantly changing conditions and the mastication changes as a result of the continuously rising temperatures in the mixer.
The filler, the softener and some auxiliary substances are incorporated within the same intermittent process after the conclusion of the mastication process whilst a cross-linking agent and other auxiliary substances are incorporated in the mixture in a further intermittent mixing operation. This is carried out in a further internal mixer after the mixture batch has cooled. In the case of a very rapidly reacting finished mixture, an accelerator is added within a further mixing operation at the lowest possible temperature.
In the present state of the art, rubber mixtures are manufactured almost exclusively in intermittent processes in the internal mixer. In the example shown in FIG. 10, it is assumed that a containing mixture is being prepared. The natural rubber is masticated in the internal mixer after appropriate crushing and is thus prepared for receiving the other constituents of the mixture.
The mixing process in the internal mixer, which acts in a manner similar to an agitator vessel, produces a continuously rising temperature, resulting in constantly changing conditions and mastication behaviour.
Moreover, considerable differences in temperature occur in the compound as a result of the disadvantageous mixing behaviour and due to the poor dissipation of heat, so that the temperature control of process is, at best, very difficult and, at worst, impossible.
The filler and the softener, together with, if appropriate, auxiliary substances, are incorporated in the same intermittent process after the mastication has been concluded. In this connection, the entire quantity of softener and, if the mixture is to contain a high content of filler, a portion of the filler are added to the batch.
The constant redistribution in the internal mixer and the, by now, even more sharply rising temperature cause the incorporation of the filler into the mixture and subsequently the dispersion of the filler agglomerates. Since the dispersal process becomes increasingly impaired as the temperature rises, and the temperature rises more rapidly as the proportion of filler added to the mixture increases, the poor heat transference characteristics of the mixer means that the temperature cannot be kept constant therein. Accordingly, in practice, only a limited quantity of filler can be incorporated in each batch.
In consequence, when the desired filler content is to be high, the batch has to be subjected to further processing in a further intermittent mixing process in the internal mixer after the mixture has been cooled. In this further step, the remaining quantity of filler is incorporated into the mixture. It is sometimes necessary to carry out four or five cooling and remixing steps to incorporate the desired amount of filler into the mixture. The finished batch is subsequently rolled and is stored in the form of strips or in granular form for further processing.
The cross-linking agents are incorporated in the mixture in a further intermittent process in the internal mixer. The reaction is accompanied by a rise in temperature but this latter must not be allowed to exceed a pre-determined value.
Particularly in the case of rapid reactions, this can necessitate a plurality of mixing processes to introduce the cross-linking agents. It is essential that the constituents of the cross-linking system are uniformly distributed and homogenized in the batch, but this normally causes the temperature to rise rapidly in a detrimental manner.
For this reason, the conventional manufacture of a rubber-containing mixture can necessitate seven or eight separate mixing processes, with the consequential necessity for large storage capacities and a high expenditure on both machinery and personnel.